The general aim of this project is to elucidate the mechanism of action of cardiac glycosides at the cellular and the molecular levels. Since there is increasing evidence that the Na ion, K ion-ATPase complex of the cell membrane is the receptor for the toxic and the therapeutic effects of cardiac glycosides, our attention is focused on the interactions of these drugs with this enzyme complex. The specific research projects are as follows: Kinetics of interaction of cardiac glycosides with the Na ion, K ion-ATPase of intact cells and tissue will be studied. The intact human red cell will be used as a first model for such studies. These cells will be exposed, in vitro, to commonly used cardiac glycosides (ouabain, digoxin and digitoxin), and the rate of inhibition of the enzyme intact cells will be determined. The effects of various extracellular ligands of physiologic and pharmacologic importance (e.g., Na ion, K ion, Mg2 ion, Ca2 ion, pH, inorganic phosphate, and several drugs on the kinetics of inhibition of the enzyme of the intact cell) will be studied. The intracellular environment of the intact cell will also be altered, and the effects of these alterations on the rate of inhibition of the enzyme will be studied. Similar experiments will be performed to determine the influence of extracellular and intracellular environments on the rate of regeneration of the enzyme of the intact cell. The possibility that the enzyme of the intact red cell may be inhibited in patients who are continuously exposed to therapeutic doses of these drugs will also be investigated. An attempt will be made to develop methods for the study of the kinetics of interaction of cardiac glycosides with the enzyme of intact heart. BIBLIOGRAPHIC REFERENCE: W. Huang and A. Askari, "Transport ATPase of Erythrocyte Membrane: Sensitivities of Na ion, K ion-ATPase and K ion-Phosphatase Activities to Ouabain" Arch. Biochem. Biophys. 175, 185, 1976.